The biosynthesis of folic acid and certain aspects of its function as a coenzyme are under investigation. The individual enzymes involved in the overall conversion of GTP to the pterin portion of folic acid (i.e. 6-hydroxymethyl-7,8-dihydropterin) have been purified and some of their properties and the reactions they catalyze have been investigated. In addition the enzymatic conversion of GTP to L-threo-dihydroneopterin is under investigation. The latter compound occurs in large quantities in E. coli, but its biochemical function is unknown. Recently a mutant of E. coli has been isolated that requires L-threo-neopterin for growth. Further studies of this mutant are underway with the goal of obtaining information about the biochemical role of L-threo-neopterin. Two GTP cyclohydrolases have been discovered in E. coli. The two have been separated from one another and their properties established. One is concerned with the biosynthesis of folic acid and it is felt that the other may be concerned with the biosynthesis of riboflavin. A mutant of E. coli has been isolated that grows in the complete absence of folic acid. We have shown that this mutant initiates protein synthesis with methionyl-tRNA rather than the normal formyl-methionyl tRNA.